1. Field of the Invention
In one aspect this invention relates to shell molding. In a further aspect, this invention relates to a method of making shell molds. In yet a further aspect, this invention relates to shell molds used in the shell molding process.
2. Description of the Prior Art
Shell molding methods and equipment were introduced in the United States about 20 years ago. Since then shell molding has gained a wide acceptance, and today thousands of parts are produced using the shell molding process. The process allows the production of cast articles having a good surface finish but without the cost inherent in forming an investment cast article.
Generally, shell molding consists of making a pattern which can be heated, the patterns normally being metal. The pattern is heated to an elevated temperature on the order of 400.degree. F or higher and then coated with a sand-resin mixture, such as ordinary silica sand coated with phenol-formaldehyde resin. The heat from the heated pattern causes an initiating or curing agent present in the resin to cure the resin to a hard thermoset material bonding the sand grains into a self-supporting mold.
The side of the mold which contacts the pattern will be fully cured by the heat present in the pattern into a shape suitable for use as the interior of the mold. Resin in the portion of the mold furthest away from the heated pattern will melt into a thermoplastic material which adheres the grains together. The molds which are normally made in two mating pieces, are usually passed through a radiant heating area such as a gas or electric furnace which cures the thermoplastic resin on the back portion of the mold. The two halves of the pattern can then be cemented together and cured to form a cavity suitable for receiving molten metal.
In general, the shell molds used in the prior art have a buildup of at least one-fourth inch, many of the molds being 1/2 inch or more. A thick mold is required to hold the cast liquid metal in position within the mold during solidification of the metal since the resin present which bonds the sand will carbonize and eventually decompose under the intense heat of the molten metal.
The thickness of the prior art shell mold is undesirable in that the cost of making molds increases with increasing thickness. The primary cost factor is the resin used to bond the sand. Resin is considerably more expensive than sand. The greater the amount of sand used, the greater the amount of resin which is also used. In addition, thick molds require greater amounts of heat to cure the resin binder. Also, the thicker molds require longer curing times.